At 11:56 PM 7/6/2005, Tom Rauch wrote:
> > One way is if you have an air core inductor that you can
>measure the loss
> > of (say you've got it resonated with a capacitor at 14
>MHz). Shove the
> > suspect hunk of plastic in the middle and see if the Q of
>the coil
> > drops. If not, you're good to go.
>
>Space winding an inductor reduces turn-to-turn capacitance
>and increases inductor Q. Fill the area between turns in
>with a dielectric and Q decreases. That's because
>capacitance and circulating currents increase. Knowing that,
>how do we quantify how much Q loss is caused by an increase
>in turn-to-turn capacitance as a dielectric is brought near
>or against the turns and how much is lost through power
>dissipation in the dielectric? How many people can measure
>Q?
I think one could get a "quick and dirty" feel using a typical antenna
analyzer and measuring the SWR bandwidth of the coil and capacitor combination.
>How does placing the PVC in a strong magnetic field test
>characteristics as a dielectric in a strong electric field?
As you point out, space wound coils have signficant self C (upon which
tesla coilers depend). It's meant as a qualitative test, not as a
precision dielectric properties measurement scheme, and at least it's at
the frequencies of interest (as opposed to 2.5 GHz).
My experience has been that PVC pipe is either just fine or really, really
bad. Most of the time it's ok, but every once in a while you get a piece
that has one of the ills I described (metallic or carbon black or lossy
fillers). Same also applies to cardboard tubes/concrete forms. Most are
ok, some are really, really bad.
>I think the only valid test would be to make a stub from the
>PVC exactly as planned and measure the impedance of the
>stub. Since the choking ability of the "Bazooka" is
>determined by the operating frequency impedance of the open
>end of stub, and since anything that lowers the impedance of
>the stub or increases losses in the stub formed between the
>outside of the coax shield and the inside of the sleeve also
>reduces choking impedance, even the best dielectric would
>reduce effectiveness of the sleeve choke.
This is true. But before one goes to all the trouble to make the stub,
etc., a quick 2 minute test to see if it's even worth starting the project
might be worthwhile.
>The sleeve needs to have largely air dielectric between it
>and the coax shield. The coax shield has to be small outside
>diameter compared to the inside diameter of the sleeve.
>
>That's why the Isopole vertical with large open sleeve ends
>and air dielectric works so well for reducing feedline
>shield currents, and why folding the braid back for 1/4 wl
>over a PVC coax jacket makes a very poor balun.
Not that I'd want to try it, but I can conceive of using two concentric
plastic tubes. The central one's ID is just a bit bigger than the OD of
the coax. The outer one is covered with some conductive coating
(essentially "making" some sort of air/plastic dielectric coaxial
line). Yeah, there are probably "better" ways to do it, and for that
matter, there are probably better ways to do the choking in the first
place, but heck, antenna experimentation is what it's all about.
>73 Tom
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